The present invention relates generally to a process for the use of pyrrolidones for concentrating fine colloidal particles from liquid suspensions containing colloidal particles.
Control and modification of interfacial properties of colloidal particles in liquid media have long been the subject of a great deal of attention. Controlling the stability and wettability of colloidal dispersions is often the key factor that determines the efficiency of various industrial processes such as effluent treatment and mineral and ceramic processing, and the quality of products such as cosmetics, pharmaceuticals, and foods, as described by P. Somasundaran and Xiang Yu in Advances in Colloid and Interface Science, volume 53, pages 31-49 (1994).
Colloidal particles in liquid dispersion media exhibit Brownian motion resulting in frequent collisions. Stability and other dispersion characteristics are affected by the nature of the interactions between the particles during such collisions. When attractive forces dominate, the particles will aggregate and the dispersion may destabilize. When repulsive forces dominate, the system will remain in a dispersed state.
Flocculation occurs by concentrating finely divided particles which are suspended in a liquid. Generally, flocculation occurs through the utilization of an inorganic coagulant or organic flocculant that brings the particles together, as described in U.S. Pat. No. 5,330,546, issued to Ramesh et al. Inorganic flocculants, such as alum and iron salts, may be used, however water soluble organic polymers are more commonly used to flocculate particles. Naturally occurring and synthetic polymers are also used as flocculants, especially in the mining industry. The principal natural polymer flocculants, such as starch and guar, are high-molecular weight polysaccharides consisting of a mixture of linear and branched segments.
Synthetic polymers have the advantage that they can be tailored to specific applications, resulting in a wide range of commercially available flocculants of varying charge, composition, and molecular weight. The most widely used synthetic coagulants include polydiallydimethylammonium chloride (poly-DADMAC or DADMAC) and condensation polymers of epichlorohydrin and dimethylamine (Epi/DMA). These structures vary greatly in molecular weight.
Colloidal silica particles have been found to flocculate in a solution of water and 2,6-lutidine at close to the phase transition or demixing temperature, as described in D. Beysen and D. Esteve, 54 Physics Review Letters 2123 (1985). The flocculation was found to be reversible. Upon lowering the temperature below the lower critical solution temperature, the particles were re-dispersible. This flocculation phenomenon was thought to be caused by capillary condensation, i.e., the coexistence curve being displaced between two particles as a result of wetting, as described in M. P. Gelfand and R. Lipowsky, B36 Physics Review 8725 (1987). Polyvinylpyrrolidones have been used to flocculate kaolinite and montmorillonite, as described by A. M. Gad, M. A. Khattab, W. Kotb and F. F. Assaad, 34 Alexandria Engineering Journal D119 (1995) and S. Smimabayashi, M. Okuda, and M. Nakagaki, 36 Chemical and Pharmaceutical Bulletin 1257 (1988).
N-alkyl pyrrolidones have found wide commercial acceptance as non-toxic, aprotic chemical solvents. Similar to other members of the lactam family, pyrrolidones are resonance stabilized by the lactam oxygen and adjacent ring nitrogen. Among many of their advantageous characteristics are solubility in both polar and non-polar solvents and an ability to participate in hydrogen bonding. The properties of pyrrolidones have attracted increasing industrial interest for a wide variety of applications, such as solvents in cleaning printing presses, coating strippers in the electronics industry and formulating agents for many crop protection products, as described in a Technical Brochure, Specialty Pyrrolidones, BASF Corporation, Mount Olive, N.J. 07828. For example, U.S. Pat. Nos. 5,093,031 and 5,294,644, incorporated herein by reference, describe surface active properties of N-alkyl pyrrolidones, such as solubility, wetting, viscosity building, emulsifying and/or complexing. However, the use of N-alkyl pyrrolidones for the concentration of colloidal particles is heretofore unknown.
Known flocculants generally suffer from low settling rates, high sediment volume, and a difficulty in reclaiming the flocculant after sedimentation. In addition, many available flocculants are not biodegradable and emit noxious odors. Other disadvantages of a number of available flocculants include high vapor pressure and toxicity.